Ash and honeylocust plant bugs in Minnesota landscapes

Plant bugs (Family: Miridiae) are a group of insects that feed on the foliage of trees by piercing the leaf tissue with their needle-like mouthparts and ingesting the sap from the leaves. The most common tree feeding plant bugs in Minnesota are the ash plant bug (Tropidosteptes amoenus) and the honeylocust plant bug (Diaphnocoris chlorionis). The ash plant bug feeds on green, white and black ash while the honeylocust plant bug feeds on all varieties of honeylocust planted in urban landscapes. Plant bugs do not seriously injure mature, vigorously growing trees and the damage to foliage is cosmetic. However, when damage is severe it can impact the health of young, recently planted trees.

Ash plant bug

Identification

Ash plant bugs are oval and range up to 1/8" long as nymphs and ¼" long as adults. Nymphs are pale yellow/red to brown/black and look like large, mobile aphids. Adults are similar in appearance to the nymphs, but are more slender and have wings that cover the abdomen (Fig. 1). Ash plant bug nymphs and adults also produce black sticky specks of excrement on the underside of infested leaves (Fig. 2).

Figure 2. Excrement of ash plant bugs on the underside of an ash leaf.

Lifecycle:

Ash plant bugs produce two generations each year. Eggs hatch in the spring just after the ash leaves begin to expand. First generation adults appear in June; they then mate and deposit their eggs on the midribs of leaves. The nymphs that hatch from these eggs mature into adults by July or August. These adults mate and lay their eggs on twigs, bud scales and other protected places in the bark where the eggs overwinter. Second generation adults remain active until the first hard frost. While the second generation of ash plant bugs is active for a longer period of time, the first generation causes more damage to leaves. The young expanding leaf tissue in the spring is much more sensitive to the saliva that is injected into the leaves as the ash plant bugs feed; the early leaves are easily damaged by this feeding.

Injury

In early spring, nymphs feed on the new shoots, leaf stems, and the underside of leaves. Light to moderate feeding by the nymphs creates tiny yellow spots where chlorophyll has been removed from the leaves (Fig. 3). This spotting, called stippling, can merge into larger damaged areas where leaves appear mottled. The mottled leaves may wilt, turn brown and appear scorched (Fig. 4). In addition the leaves may also twist and look deformed (Fig. 5). Damaged leaves can remain on the tree until leaf drop in the fall. Ash plant bug feeding does not typically result in leaf drop in the spring. If leaves are falling in the spring it is usually due to another condition, such as anthracnose, a plant disease , http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/anthracnose/index.html.

Jeff Hahn, University of Minnesota

Figure 3. Ash plant bugs create stippling when they feed on foliage.

Jeff Hahn, University of Minnesota

Figure 4. Damaged foliage may appear scorched.

Management

James Solomon, USDA Forest Service, Bugwood.org

Figure 5. Severely damaged foliage may appear contorted.

Ash plant bug damage to trees is cosmetic; it only impacts the tree's appearance, especially when the tree is mature and vigorously growing. Plant health is not affected by ash plant bug infestations. The best course of action is to tolerate and ignore the damage.

If it is necessary to use an insecticide, apply it to the foliage when the nymphs are first active during the spring. Treatment should be made to minimize damage to the leaves. There are several options available that are affective to

Insecticidal soap is a low impact insecticide that can be effective against the nymphs. However, the spray has to come in direct contact with the nymphs to be effective; there is no residual and a repeat application may be necessary.

Residual insecticides, such as permethrin, bifenthrin, and carbaryl, are also effective against ash plant bugs. Large trees are usually best treated by a professional tree care service provider.

Caution: Read all label directions carefully before purchasing and again before using a pesticide. Only treat plants that are listed on the label. Information on the label is the law and final authority.

Honeylocust plant bug

Identification

Honeylocust plant bug nymphs and adults are oval in appearance and range up to 1/8" long as nymphs and ¼" long as adults. Both nymphs and adults are pale green and blend in with foliage. Adults have fully functional wings whereas nymphs do not (Fig. 6). Nymphs are very active and will climb onto anything that may come in contact with the foliage.

Lifecycle

Honeylocust plant bugs have one generation per year. Overwintering eggs hatch just as honeylocust leaf buds open in the spring. The newly hatched nymphs climb onto the unfolding foliage and begin feeding. They feed until June when they mate and lay eggs in clusters under the bark of young twigs. The eggs remain dormant through the remainder of the year until the following spring.

Injury

Feeding damage by honeylocust plant bugs is caused by the saliva they inject into the foliage. The saliva causes stippling and also causes tissue distortion. Damaged leaves remain small and become twisted and dwarfed (Fig. 7). The damage looks similar to herbicide injury. However honeylocust plant bug damage occurs on new foliage throughout the canopy of the plant, whereas herbicide damage tends to occur on the side where spray applications were made.

In addition, stippling is noticeable on honeylocust plant bug damaged foliage. When leaves are severely damaged, leaves may drop, but in many cases the damaged leaves will remain on the tree until the fall. This feeding damage is primarily cosmetic and does not impact the health of vigorously growing trees. However, young trees growing in stressful conditions can be impacted by the premature leaf drop, especially if it is severe.

Management

Most honeylocust plant bug damage is just cosmetic and typically does not impact the health of trees. The best approach for tree owners is to keep trees healthy and tolerate the damage.

If trees have had problems with honeylocust plant bugs in the past, check trees at bud break to determine if high numbers are present. If it is necessary to treat to protect the appearance of the tree or in the unusual case where the tree's health is at risk, a pesticide application can be used. The best timing for an insecticide application is at bud break. It may be necessary for large trees to be treated by a professional tree care service provider.

Insecticidal soap is a low impact insecticide that can be effective against the nymphs. However, the spray has to come in direct contact with the nymphs to be effective; there is no residual and a repeat application may be necessary.

There a variety of longer residual insecticides available, such as permethrin, bifenthrin, and carbaryl. Use these insecticides carefully and judiciously as they are broad spectrum and can impact non-target insects. For example, broad-spectrum insecticides can kill the predators of spider mites; this can cause a spider mite outbreak.

While systemic insecticides, such as imidacloprid and dinotefuran, are effective, they should be avoided as they are toxic to bees. Once applied, these insecticides can move into the pollen and nectar of flowers when they are blooming and bees can be exposed to them. Honeylocust flowers are very attractive to bees when they are blooming.

Caution: Read all label directions carefully before purchasing and again before using a pesticide. Only treat plants that are listed on the label. Information on the label is the law and final authority.